1Stegosaurus (steg-oh-SORE-uss) was a dinosaur that lived on Earth long ago. Its name means "plated lizard." It had 17 large, pointed plates on its back. The plates were bony and hard. They stuck up along the Stegosaurus's backbone. The largest plates that have been found were two feet tall!2 What were the plates for? They may have helped protect the dinosaur's backbone. They may have helped Stegosaurus recognize others of its own kind. They may have helped this dinosaur warm up and cool off. Inside the plates were little holes. Scientists think there were blood vessels inside the plates. On cool days, Stegosaurus may have basked in the sun, like today's reptiles do, to warm up. The plates would have heated up from the sunlight. This would have warmed the Stegosaurus's blood. On hot days, Stegosaurus may have stood in the shade. The plates and the blood inside them would have cooled down.3 This dinosaur walked on four thick, study legs. It had a small head and a short, thick neck. Scientists think its brain was no bigger than a walnut! It had a thick tail with four big spikes on the end. The spikes were used to defend it from predators. Each spike was three feet long! From nose to tail, this dinosaur was thirty feet long -- almost as long as a school bus.